Vaginal examination vs. cervical length - which is superior in predicting preterm birth?

Objective: To compare the predictive value of preterm birth (PTB) by transvaginal sonographic cervical length (CL) measurement to digital examination of the cervix (Bishop score - BS), in patients with premature contractions (PC) and intact membranes. Design: A retrospective case-control study. Setting: Meir Medical Center, Kfar Saba, Israel. Population: Women at 24-34 weeks of gestation who were hospitalized with PC and intact membranes. Methods: All patients underwent CL and BS measurements upon admission. Power analysis revealed that 375 patients were needed to show a significant difference between the two methods for predicting PTB. Each one served as her own control. Main outcome measures: PTB < 37 and < 34 weeks. Results: Receiver-operator characteristic curve (ROC) and logistic regression analyses indicated a correlation between both shortened CL and increased BS to PTB (P < 0.001). Neither test offered an advantage in predicting PTB. Areas under the curve for BS and CL ROC were similar for PTB before 37 weeks gestation (0.611 vs. 0.640, P = 0.28). For nulliparous women, CL predicted PTB better that BS (0.642 vs. 0.724, P = 0.03). For singleton and multiple pregnancy pregnancies, BS and CL did not differ significantly in predicting PTB (P = 0.9, P = 0.2, respectively). For nulliparous with multiple pregnancy, the BS and CL ROC curves differ nearly significantly (0.554 vs. 0.709, P = 0.07), with better predictive ability for CL. Conclusions: CL and BS have similar value in predicting PTB in patients with PC. For nulliparous women, CL is superior over the BS.